Aly DeMarco was just about to leave her Austin, Texas, home for school when she heard her dad called her name in a "high-pitched, girly" voice. Michael DeMarco had been riding his stationary bike in the back room. The family's youngest daughter, 4-year-old Addison, and the family dog, a beagle mix, were in the room with him.

“I thought he was probably going to ask me to watch Addison for a moment, but I really didn't want to go because I was already late for school,” Aly said. “But his voice sounded really weird, so I went back.”

Aly arrived in time to see dad tumble on the floor. He struggled to stand up, but fell down on the dog cage.

Aly said she panicked. Her mother was dropping off the family's 10-year-old daughter at school. Her little sister was standing there, and the dog, Bow, thought her father was playing so he jumped on the dad's back and licked all over his face.

“I was waiting for someone to pop out and say it was a joke, I was testing your babysitting skills,” Aly said.

But this wasn't a drill. Michael DeMarco had suffered a cardiac arrest.

Aly, who had just finished an infant CPR course so she could babysit, called her mother, but Marion DeMarco had left her cellphone at home. So Aly dialed 911 instead.

After a few questions, the operator asked Aly to perform CPR on her dad and stayed on the line encouraging her. Aly counted one, two, three with the operator and pressed her father's chest on four. She repeated the movements for 10 minutes.

"I don't know how she did it," Michael DeMarco, who is 6-foot-1 and weighs about 215 pounds, told ABC News. "Doing hand compression on an adult is not easy for a 12-year-old."

Michael DeMarco was later rushed to St. David's Medical Center, where he was in a coma for six days.

When he finally woke up, DeMarco was confused as to who saved him.

“When I came out of the coma and they told me [what happened], I had to ask several times, ‘Wait! What happened?’” Michael DeMarco said.

Michael DeMarco knows he is alive today thanks to his daughter’s quick response.

“If I walked out 10 minutes before, he'd be stuck there with my 4-year-old sister," said Aly, who has since turned 13. "That would be horrible... I wouldn't have him today. It makes me feel really good that I did something like that."